Method of manufacturing a compound steel material of a high corrosion resistance

ABSTRACT

A compound steel material of high corrosion resistance is manufactured by alloying a carbon steel carrier material having a carbon content of at most 0.20C and of at least 0.04C with a carbide and nitride forming substance, compounding the alloyed carrier material with a ferritic chromium steel material of normal carbon content, followed by hot-rolling the compound steel material to a hot rolled strip or sheet which is then annealed at a temperature and for a time period sufficient for the carbon content of the ferritic chromium steel coating layer to be reduced to between 0.001 and 0.003% so as to increase the corrosion resistance of the chromium steel material to that of a superferritic material. After annealing, the sheet may be etched and surface-finished or cold-rolled, recrystallization annealed and thereafter surface-finished, such as temper-rolled. The ferritic chromium steel material has at most 0.1% of carbon prior to annealing. The carbide and nitride forming substance may be titanium whose content in the alloyed carrier material is preferably between 0.50% and 2.00%. The thickness of the ferritic chromium steel coating layer is between 50 and 500 μm, the annealing temperature between 650° and 900° C., and the annealing time period between 8 and 72 hours.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing a compoundmaterial, and more particularly to a method of manufacturing such amaterial which has a high corrosion resistance.

The corrosion resistance of ferritic chromium steel materials is, inaddition to the chromium content and the addition of, for instance,molybdenum, determined by the carbon and nitrogen content in the steelmaterial. While the conventionally manufactured ferritic chromium steelAISI 430 corresponding to SAE 51430 has a lower resistance to oxidizingand reducing acids, bases, and sulfur dioxide and chlorine containingatmosphere than an austenitic chromium-nickel steel AISI 304corresponding to SAE 30304, its corrosion resistance at the samechromium content improves with the reduction of the carbon and nitrogencontent. When the carbon content is between 0.001 and 0.003% and thenitrogen content is not exceeding 0.01%, this chromium steel(superferrite) has corrosion resistance values which are higher thanthose of the austenitic chromium-nickel steel. In particular, theresistance of this superferritic steel to stress corrosion cracking,intergranular, pitting and crevice corrosion is better than theresistance of the austenitic alloyed steel. Also, thecorrosion-resistance in oxidizing and reducing acids, as well as in theatmosphere, is higher than that of the austenitic chromium-nickel steel.

Ferritic chromium steel materials with reduced carbon contents of 0.001to 0.003% could, according to the recent state of the melting andrefining techniques, be manufactured only in electron beam vacuum ovens.The other improved manufacturing methods, such as the vacuum oxygenblow-refining method, the vacuum inductive melting method (VIM) and theargon-oxygen decarburization method (AOD) produce steel materials ofcarbon contents of between 0.01 and 0.02%. In order to give the "extralow carbon" steel materials having 0.01 to 0.02% of carbon content, acorrosion resistance which equals or exceeds the corrosion resistance ofthe austenitic steel materials, the carbon and nitrogen contents of thesteel material must be chemically reacted, that is, stabilized. In mostinstances, titanium is used for the stabilization, but other alloyingelements, such as niobium and tantalum can also be used for the samepurpose.

Experience and tests with ferritic chromium steel materials which arestabilized with titanium have shown that such materials have asubstantially higher corrosion resistance than non-stabilized chromiumsteel materials having a normal carbon and nitrogen content (such as SAE51430 steel). The titanium content must be at least six times higherthan the sum of the carbon and nitrogen contents. The ferritic chromiumsteel materials which are stabilized with titanium, however, aredisadvantageous in comparison with the non-stabilized materials in thatthey have a lower purity and have a surface of worse quality. Thesurface cannot be ground to give it a mirror-like appearance.

It has also been recently proposed, in German Pat. No. 26 21 329, tocompound a carrier steel material of deep-drawing grade, which isalloyed with a carbide and nitride forming substance, with a ferriticchromium steel material of normal carbon content, to roll the compoundsteel material to a strip of sheet, and then to anneal the compoundsteel material of the fine sheet at a temperature and for a time periodsufficient for the carbon content of the ferritic chromium steel coatinglayer to be reduced to between 0.001 and 0.003%; thereby increasing thecorrosion resistance of the ferritic chromium sheet material to that ofa superferritic material. However, experience has shown that, under manycircumstances, it is difficult to perform the annealing operation.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to avoidthe disadvantages of the prior art.

More particularly, it is an object of the present invention to devise amethod of producing a compound material which is provided with achromium steel coating layer at one or more of its surfaces, wherein thecorrosion resistance of the chromium steel layer corresponds to that ofa superferritic material.

A further object of the present invention is to present a method ofmanufacturing a corrosion-resistant compound material without resortingto the use of non-stabilized superferritic materials.

In pursuance of these objects and others which will become apparenthereafter, one feature of the present invention resides, briefly stated,in a method of manufacturing a compound steel material having at leastone chromium steel coating layer, which comprises the steps of alloyinga carbon steel carrier material with a carbide and nitride formingsubstance; compounding the alloyed carrier material with a chromiumsteel material of normal carbon content; hot-rolling the compound steelmaterial to a sheet or strip material; and increasing the corrosionresistance of the chromium steel material to that of a superferriticmaterial, including annealing the compound steel material at atemperature and for a time period sufficient for the carbon content ofthe ferritic chromium steel coating layer to be reduced to between 0.001and 0.003%.

DETAILED DISCUSSION OF THE PREFERRED EMBODIMENT

As mentioned above, it is an object of the present invention to obtain amaterial which has the same or a higher corrosion resistance than anaustenitic chromium-nickel steel SAE 30 304, which has the followingcomposition:

    ______________________________________                                        C max Si max   Mn max   P max S max                                           %     %        %        %     %      Cr    Ni                                 ______________________________________                                        0.07  1.00     2.00     0.045  0.030 17-20 8.50-10                            ______________________________________                                    

To achieve this objective, the present invention starts with a ferriticchromium steel according to a SAE 51 430, which has the followingcomposition:

    ______________________________________                                        C max Si max   Mn max   P max S max                                           %     %        %        %     90     Cr    Ni                                 ______________________________________                                        0.010 1.00     1.00     0.045 0.030  15.50-                                                                        17.50                                    ______________________________________                                    

This ferritic chromium steel material is compounded with a carriermaterial which is alloyed with carbide and nitride forming elements, inparticular titanium, whereupon the compound material is hot-rolled intosheets and subsequently annealed until the carbon content in theferritic chromium steel coating layer is reduced to between 0.001 and0.003%.

When the method is performed in this manner in accordance with theinvention, there is obtained a facilatation of the annealing operation,inasmuch as there is no need to worry about oxygen influences on andthus about scaling of the surface region.

The thickness of the hot-rolled sheet or strip material may correspondto that of course, intermediate or fine sheet material, the latter beingpreferred.

The carrier material which is used to form the compound material is acarbon steel having a carbon content of at most 0.20% and of at least0.040% C. Preferably the carrier material used is a plain carbon steelor a low-alloyed carbon steel.

On the other hand, the carbon content of the ferritic chromium steelmaterial which is to be decarburized should not be above 0.1%, and ispreferably between 0.05 to 0.07%.

The titanium content in the alloyed carrier material is preferably inthe range between 0.50% and 2.00%. Preferably, the thickness of thechromium steel coating layer to be decarburized is between 50 and 500μm.

The applicable annealing temperature are preferably in the regionbetween 650° and 900° C., while the annealing time periods can bebetween 8 and 72 hours depending on the titanium content of the carriermaterial, the thickness of the coating layer, and the selectedtemperature. Under certain circumstances, even substantially shorterannealing time periods are necessary.

The decarburization of the ferritic chromium steel coating layer takesplace on the basis of diffusion. The advantage of this decarburizingprocedure resides in the fact that only a very thin layer of the coatingmaterial is to be treated and that the treatment is accomplished afterthe termination of a strip or sheet manufacturing process. After theannealing, the fine sheet or strip material is dressed and adjusted asusual in the customary alloyed steel manufacturing process. According tothis method, there can be obtained carbon contents in the plating layerof 0.001 to 0.003%, which gives the ferritic chromium steel a corrosionresistance which is higher than that of the austenitic chromium-nickelsteel. The obtained decarburized, non-stabilized chromium steel layer isdevoid of carbide inclusions and impurities and thus the exposed surfacethereof can be polished to high luster (highly lustrous polishedsurfaces possess an even better corrosion resistance). Thedecarburization of thin alloyed steel layers in accordance with thediffusion process is also possible in connection with sheets ofintermediate and large thickness. The decarburization is independent ofthe chromium content of the plating layer. The corrosion resistance ofthe plating layer is improved with an increasing chromium content and byaddition of molybdenum.

After the decarburizing annealing operation, the hot-rolled sheet orstrip material can be used for some applications after being merelyetched and surface-finished, for instance, temper-rolled. However, underusual circumstances, the hot-rolled sheet is cold-rolled, afterwardsrecrystallization annealed, and surface-finished, such as temper-rolled,so as to obtain good mechanical properties.

Preferably, a material is used as the starting compound material inwhich the carrier material alloyed with titanium, in its molten state,is poured on a ferritic chromium steel sheet in a compounding process,the chromium sheet having been introduced into an ingot mold prior tothe pouring or molding of the carrier material.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofprocesses differing from the types described above.

While the invention has been illustrated and described as embodied in amethod of manufacturing a compound steel material of high corrosionresistance, it is not intended to be limited to the details shown, sincevarious modifications and structural changes may be made withoutdeparting in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

The compound steel material can be formed either by a compound castingprocess by a cladding process for example a preliminary hot rolling ofthe two steel partners of the compound or by explosion-cladding. It goeswithout saying that the round material can be manufactured in any othersuitable way, for example welding.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. A method of manufacturing acompound steel material having at least one chromium steel coatinglayer, comprising the steps of alloying a carbon steel carrier materialwith a carbide and nitride forming substance; compounding the alloyedcarrier material with a chromium steel material of normal carboncontent; hot-rolling the compound steel material to a sheet; andincreasing the corrosion resistance of the chromium steel material ofthe sheet to that of a superferritic material, including annealing thecompound steel material at a temperature and for a time periodsufficient for the carbon content of the ferritic chromium steel coatinglayer to be reduced to between 0.001 and 0.003%.
 2. A method as definedin claim 1, wherein the carbon content of the ferritic chromium steelmaterial prior to said increasing step is at most 0.1%.
 3. A method asdefined in claim 2, wherein the carbon content of the ferritic chromiumsteel material prior to said increasing step is between 0.05 and 0.07%.4. A method as defined in claim 4, wherein the titanium content in thealloyed carrier material is between 0.50% and 2.00%.
 5. A method asdefined in claim 1, wherein the thickness of the ferritic chromium steelcoating layer is between 50 and 500 μm.
 6. A method as defined in claim1, wherein the annealing temperature is between 650° and 900° C.,depending on the thickness of the coating layer.
 7. A method as definedin claim 1, wherein the annealing time period is between 8 and 72 hours,depending on the carbide and nitride forming substance content in thecarrier material, on the thickness of the coating layer, and on theannealing temperature.
 8. A method as defined in claim 1; and furthercomprising the steps of etching and surface-finishing following saidannealing step.
 9. A method as defined in claim 1; and furthercomprising the steps of cold-rolling, recrystallization annealing andsubsequently surface-finishing, following said annealing step.
 10. Amethod as defined in claim 1 wherein the carbon content of the carriermaterial is at most 0.20%.
 11. A method according to claim 1 wherein thecarbon content of the carrier material is at least 0.04%.